his courage, or his malice, to the sticking place. Perhaps he regarded the first seven occasions partly as practice runs. Perhaps he would have crept into the old man's chamber a hundred or a thousand times without incident if the old man had remained soundly asleep each time. Perhaps he killed the old man the first time he went into his room and is lying or deluded about the seven times before that.

This, surely, is the difficulty with Poe's unreliable narrators, and particularly with a man who is clearly mad and spends much of the story eagerly assuring us that he is not. We have to decide when to believe him and, when we refuse to do so, work out for ourselves what the truth might be.

In Poe's "The Tell-Tale Heart," it may be that the narrator does not kill the old man during the day because he cannot abide to look at the eye.

It is impossible to say how first the idea entered my brain; but once conceived, it haunted me day and night. Object there was none. Passion there was none. I loved the old man. He had never wronged me. He had never given me insult. For his gold I had no desire. I think it was his eye! yes, it was this! He had the eye of a vulture—a pale blue eye, with a film over it. Whenever it fell upon me, my blood ran cold; and so by degrees—very gradually—I made up my mind to take the life of the old man, and thus rid myself of the eye forever.

The narrator is clearly insane: he admits that he has no reason to kill the old man other than his eye. His description of the eye personifies it into a living thing, separate from the man, for he is aware that the old man has done him no harm. He does feel, however, threatened by the eye. He describes it as an eye much like that of a vulture. The association with these birds is death because they are scavengers that often survive by eating dead things—such as animals killed on the road or the carcass of another animal's kill.

Their eyes are wide and often seem to bulge. The narrator's mind has erroneously conceived that the eye itself is a threat to him.

The fact that the speaker describes the eye when it is opened would lead us to presume that he will feel less threatened if he kills the old man when he is asleep and his eye is closed. Although the white film on the eye indicates that the eye is incapable of sight, the danger in the narrator's mind may be in his perception of being seen by the eye—even though the eye cannot see.

Of course, the other reason he kills the old man at night is because he wants his actions to go unnoticed. Generally speaking, few people would be up and walking about on the streets or awake in their homes at such a late hour, limiting the number of potential witnesses to any suspicious sounds emanating from the house during the murder. 

The mentally unstable narrator claims that he is not a madman and proceeds to describe the way that he carefully, methodically planned and executed the murder of the defenseless old man he was living with. After confessing that he genuinely loved the old man and was not interested in murdering him for his money, the narrator admits that the old man's pale blue vulture eye motivated him to commit murder. For the first seven nights, the narrator peeks his head into the old man's room around midnight while he is sleeping and quietly opens his lantern so that a thin ray of light casts upon the old man's face. The narrator mentions that he refrained from murdering the old man for the first seven nights because the old man's eyes were closed. Since the old man's vulture eye is the primary feature that incites the narrator's rage and bloodlust, he is not motivated to kill the old man on any of the first seven nights, because he cannot see the old man's horrific eye. However, on the eighth night, the narrator opens his lantern and the light shines onto the old man's "Evil Eye," which inspires the narrator to attack and murder the old man.

The narrator cannot kill the old man for the first seven days because his eye is closed, and he only wants to kill him because of the Evil Eye.

The narrator peaks his head in the old man’s room very, very carefully so the old man won’t see.  He does not want to kill the old man, but he has to because of his evil eye.

And this I did for seven long nights—every night just at midnight—but I found the eye always closed; and so it was impossible to do the work; for it was not the old man who vexed me, but his Evil Eye. 

Of course, the narrator is crazy.  He keeps telling us he isn’t, but he clearly is.  Anyone who wants to kill a perfectly innocent old man just because he thinks his eye is evil has some mental instability.

The narrator describes his attempts and readiness to commit the murder. He creeps into the old man's room just at midnight, for seven nights in a row, though he finds that he cannot bring himself to do the deed as he intends. He says that each time he creeps into the room, he sees that:

[...] the eye [is] always closed; and so it was impossible to do the work; for it was not the old man who vexed [him], but his Evil Eye.

The narrator of the story does not hate the old man -- in fact, he says that he loves the old man! -- he only hates the eye. If the eye is not open, then the narrator is not incited to feel the rage and the fear that he needs in order to kill the man. However on the eighth night, the narrator says, the old man's eye:

[...] was open -- wide, wide open -- and I grew furious as I gazed upon it. I saw it with perfect distinctness -- all a dull blue, with a hideous veil over it that chilled the very marrow in my bones [...].

Only the sight of the old man's "vulture" eye can drive the narrator to the extreme emotional lengths he must feel in order to go through with the murder. When he could not see the eye, he could not kill the man, but now that the eye is visible to him, his adrenaline begins to pump and he can.